“Heart patients can work with their doctor to figure out what intensity is best for them to make the heart stronger but not at an intensity that puts the heart at risk,” McCall says. “A properly set up monitor can make their workout a safe, yet effective one.”

Although some group exercise classes have participants check their heart rates manually by feeling the pulse in the wrist or carotid artery in the neck, it ends up being only a rough estimate. You have to stop during exercise to count your pulse, which disrupts both your workout and your heart rate, plus there's lots of room for error.

To get the biggest fitness bang for your exercise buck, physiologists recommend that most healthy adults exercise in their target heart range or at about 60 percent to 85 percent of their maximum heart rate. Your maximum heart rate is the highest pulse rate or number of heartbeats you can attain in one minute of all-out effort. In order to calculate your target heart rate, you first have to determine your maximum heart rate. There are several way to do that.

The most precise method is to have a maximal oxygen uptake/maximum heart-rate test conducted at an exercise physiology laboratory administered by certified physiologists with a physician present. But this costs hundreds of dollars.

The easiest and least expensive way to estimate your maximum heart rate is to use documented formulas. Based on research of thousands of human responses to exercise testing, they provide a fairly close approximation.

The most commonly used formula is to subtract your age from 220. So if you are 50 years old, your maximum heart rate would be about 170. To find your target heart-rate zone, multiply your maximum heart rate by .60 to find the low end of the zone and multiply your maximum heart rate by .85 to find the high end of your target zone.

So, for a 50-year-old person with a maximum heart rate of 170, the target heart rate would be between 102 (170 x .60) and 144 (170 x .85).

If you want to skip the formulas to calculate maximum heart rate, the talk test is a valid method to estimate your upper aerobic limits. When you're exercising to the point of huffing and puffing and can no longer comfortably carry on a conversation, that's your aerobic limit. Set your monitor to beep or sound an alarm at that point, so you know not to exceed that intensity for too long. However, as you get more fit and your aerobic endurance improves, your upper limit will increase, and you'll need to adjust the alarm on your monitor.

McCall recommends that about 90 percent of a workout should be in the target zone, allowing for warm-up and cool-down time at the beginning and end of exercise.

The most common type of heart-rate monitors feature a transmitter strap that is worn around the chest against bare skin (women wear it just below their jogging bra) and a wrist monitor which looks like a watch. The electrodes in the transmitter strap pick up signals from your heart and send them wirelessly to the watch. These signals are continuously displayed.